iOS 18.4 beta 1 and 2 do not fix the issues.

I realised selection.bounds(at: page) delivers more correct results than page.characterBounds(at: index). But not as good as characterBounds(at) for iOS 17 and earlier.

// page: PDFPage

for (index, character) in page.enumerated() {
        
    let range = NSRange(location: index, length: 1)
    guard let selection = page.selection(for: range) else { continue }

    let charBounds = selection.bounds(for: page)
}